1. Field of the Invention
The present invention relates to a tracking control device for an optical pickup which optically reproduces information from a record medium, and in particular to a tracking control device for an optical pickup which has an improved structure for producing a tracking error signal used for tracking control.
2. Description of the Background Art
Recently, a method called a phase difference method or a DPD (Differential Phase Detection) method has received particular attention as a method for obtaining a tracking error signal which is used for tracking control of an optical pickup with respect to information pits formed at an information record surface of an optical disk. This phase difference method is specifically described, for example, in Japanese Patent Publication No. 7-105052 (1995). The phase difference method disclosed in this publication utilizes such a fact that, when a light beam radiated to the optical disk pass over information pits, a diffraction pattern, which is a map on photo detectors, changes in accordance with a shift of the light beam from the center of information pits. In the conventional tracking control device disclosed in this publication, to be brief, the photo detector has light receiving regions which are divided in a direction parallel to the tracking direction of the map of the information pits. Levels of output signals corresponding to the quantities of received light at the divided light receiving regions change in different manners corresponding to the directions and quantities of shift of the light beam from the information pit center, respectively. Therefore, the output signal sent from each light receiving region is binarized, and sampling is performed to determine which one of the binary signals changed prior to the other as well as the time difference in level change between these signals. Thereby, the tracking error signal representing the direction and quantity of shift of the light beams is obtained.
Referring to FIGS. 22 to 30, the conventional phase difference method disclosed in the above publication will be described below more specifically. In these figures, reference numbers and characters are different from those in the publication.
FIG. 22 schematically shows a structure of an optical pickup disclosed in Japanese Patent Publication No. 7-105052. Referring to FIG. 22, light beam emitted from a semiconductor laser light source 101 is reflected by a first surface 103 of a light divider 102, and passes through an objective lens 104. The light beam is then gathered to form a minute spot on an information record surface of a record medium, i.e., an optical disk 105. The light beam reflected by optical disk 105 passes through objective lens 104 as well as first surface 103 of light divider 102, and then is diffracted to change its direction by a grating 107, which is formed at a portion of a second surface 106 of light divider 102. The light beams in the changed direction are totally reflected between first and second surfaces 103 and 106 of light divider 102, and reaches a photo detector 108.
FIG. 23 shows a structure of grating 107 and a photo detector in the optical pickup in FIG. 22.
Grating 107 is divided into regions 109, 110 and 111 as shown in FIG. 23. Light beams diffracted at these regions reach photo detector units 112, 113, 114 and 115 of photo detector 108. When a distance between objective lens 104 and optical disk 105 varies, variations occur in quantities of light beam, which is diffracted at region 109 of grating 107 and reaches photo detector units 112 and 113. By calculating a difference between received light quantities at photo detector units 112 and 113, therefore, it is possible to obtain a focus error signal indicating deviation in focusing direction between the light spot and the information record surface of optical disk 105.
In the structure wherein information in optical disk 105 is recorded on information tracks formed of rows of information pits having concavities and convexities, the diffraction pattern of light which occurs when the light spot passes over the information pits can be utilized to obtain a tracking error signal, which indicates a positional deviation on the information record surface between the light spot and one of the information tracks in a direction across the information track.
FIGS. 24A and 24B, FIGS. 25A and 25B, and FIGS. 26A and 26B are diagrams showing first, second and third variation in farfield pattern depending on relative position between the light spot and information pits, which are disclosed in Japanese Patent Publication No. 7-105052, respectively. FIGS. 24A, 25A and 26A show examples of change in relative position between the light spot and information pit, and FIGS. 24B, 25B and 26B show examples of change in farfield pattern, i.e., distribution pattern of the intensity of light reflected from the light spot on optical disk 105, which depends on changes in the relative position shown in FIGS. 24A, 25A and 26A, respectively.
As shown in FIG. 25A, when a light spot 124 formed by the optical pickup moves through centers of information pits, the intensity distribution pattern changes while keeping a laterally symmetrical form as shown in FIG. 25B. If light spot 124 moves through positions shifted from the centers of information pits 125 as shown in FIG. 24A or 26A, the intensity distribution pattern cannot keep the lateral symmetry as shown in FIG. 24B or 26B, and a time difference (phase difference) occurs in change in intensity distribution pattern. The quantities of reflected light at the left and right portions of the intensity distribution pattern are converted into electrical signals by photo detector units 114 and 115 in photo detector 108, respectively, and signal processing is performed to detect the time difference between them, so that the tracking error signal is obtained.
FIG. 27 shows a circuit structure of the tracking control device disclosed in the foregoing Japanese Patent Publication No. 7-105052. FIGS. 28A-28H are timing charts showing an example of an operation of the circuit shown in FIG. 27. FIGS. 29A-29H are timing charts showing another example of the operation of the circuit shown in FIG. 27. FIGS. 30A-30F are timing charts showing still another example of the operation of the circuit shown in FIG. 27.
Signals S1-S7 at various portions in FIG. 27 as well as a tracking error signal TES are shown in FIGS. 28A-28H and 29A-29H, respectively. FIGS. 30A-30F show signals S3-S7 and tracking error signal TES in FIG. 27, respectively.
FIGS. 28A-28H show changes with time, which occur when light spot 124 moves over the information tracks on optical disk 105 from the left side toward the right side in the radial direction of the disk, and in other words, light spot 124 moves over information pits 125 from the state shows in FIGS. 24A and 24B through the state shown in FIGS. 25A and 25B to the state shown in FIGS. 26A and 26B.
In FIG. 27, current signals corresponding to the quantities of light received and detected by photo detector units (which are referred to as "PDU" in the drawings) 114 and 115 are converted into voltage signals S1 and S2 shown in FIGS. 28A and 28B by current-voltage converters (i/v) 116 and 117, respectively. Voltage signals S1 and S2 are converted into binary signals S3 and S4 shown in FIGS. 28C and 28D by binarizing circuits 118 and 119, respectively. In this case, the foregoing tracking error signal can be detected by detecting the phase difference which is the time difference in rising or falling between binary signals S3 and S4. In the above publication, the time differences in rising between binary signals S3 and S4 is detected by D-flip-flops 120 and 121. D-flip-flops 120 and 121 issue from their output terminals Q output pulses S5 and S6, of which pulse widths from rising to falling correspond to the time difference. The pulses representing such a time difference are shown in FIGS. 28E and 28F. The time difference pulses S5 and S6 are converted by a difference detector 122 into a pulse width modulation signal S7 shown in FIG. 28G. Signal S7 is converted by a low-pass filter (LPF) 123 into an analog tracking error signal TES shown in FIGS. 28H.
The right and left ends of the abscissas in FIGS. 28A-28H represent shifting of light spot 124 from the center of information pit 125. When light spot 124 is shifted from the center of information pit 125, signals S1 and S2 sent from photo detector units 114 and 115 cause not only change in time difference but also change in frequency representing the frequency of occurrence of change. In order to prevent a failure in detection of the time difference, therefore, the structure in the above publication employs D-flip-flops 120 and 121 for detecting the time difference. According to the prior art in this publication, each of D-flip-flops 120 and 121 has a terminal T serving as a clock input terminal and a reset input terminal arranged at the bottom thereof and bearing no reference character. When the reset input terminal is at a logical level L, an output terminal Q is unconditionally at level L. When the reset input terminal is at level H, the logical level equal to that applied to input terminal D is applied to terminal Q at the time of rising of the signal on clock input terminal T.
According to the art in the above publication, however, a failure in detecting the phase difference which is the time difference can be prevented only in an extremely limited state as will be described below. In the structure shown in FIG. 27, the time difference indicating the tracking error is detected based on the falling of binary signals S3 and S4. When a level change occurs in one of the binary signals shown in FIGS. 28C and 28D while the other signal is at level L, this structure in FIG. 27 does not erroneously detect the time difference in response to this change, and D-flip-flops 120 and 121 do not issue pulses S5 and S6, respectively. However, when the level change occurs in one of binary signals S3 and S4 while the other is at level H, this situation is erroneously detected as a time difference, which significantly affects tracking error signal TES. It is assumed that minute abnormal waveforms encircled by broken lines in FIGS. 29A and 29B occur at signals S1 and S2 due to a physical defect on the optical disk surface, externally applied electrical noises or an influence by information pits on a neighboring track. In this case, glitches indicated by arrows in FIGS. 29C and 29D occur at signals S3 and S4, respectively. The glitches enlarge the outputs of D-flip-flops 120 and 121 which are, in other words, pulse widths of pulse signals S5 and S6 in FIGS. 29E and 29F indicating the time difference as well as the pulse width of signal S7 in FIG. 29G, and more specifically enlarge these widths from the positions represented by broken lines and indicating the time differences corresponding to the original tracking error to the positions represented by solid lines and determined due to the glitches in the direction represented by arrows. These pulse widths can be apparently larger than the glitches themselves. As a result, tracking error signal TES in FIG. 29H issued from LPF 123 has a larger amplitude which is represented by a solid line and is enlarged in the directions indicated by arrows from the original amplitude represented by a broken line, and is significantly disturbed compared with signal TES in FIG. 28H.
Particularly, this influence increases when an abnormality occurs at the foregoing signal waveform while light spot 124 is precisely tracking the row of information pits 125. As shown in areas around centers of the abscissas in FIGS. 29A-29H, binary signals S3 and S4 change with the completely same phases when light spot 124 accurately tracking the row of information pits 125. Therefore, there is not time difference (phase difference) in rising and falling between these signals, and pulses S5 and S6 exhibiting a time difference are not generated so that tracking error signal TES maintains 0. However, if abnormalities occur at signals S3 and S4, which are original signals used for detecting the time difference, due to the foregoing reason, pulse signals S5 and S6 exhibiting the time difference much wider than the glitch itself are generated at and after the time of occurrence of the glitch as shown in FIGS. 30A-30F, and tracking error signal TES is disturbed to a large extent as shown in FIG. 30F. As described above, tracking error signal TES is originally 0 in this case, and light spot 124 accurately tracks the row of information pits 125. Thus, a so-called on-track state is achieved in this case. In spite of this, only a minute abnormality in the signal waveform causes a large error in tracking error signal TES. Therefore, the position of light spot 124 is significantly shifted from the position in the on-track state, so that accurate reproduction of information cannot be performed, and finally light spot 124 may completely be shifted away from the track.
In the conventional tracking control device, as described above, the glitch occurs due to a defect on the optical disk, externally applied noises, an influence by information pits 125 on a neighboring track. When the time difference is erroneously detected due to the glitch, this affects tracking error signal TES to an extent larger than that by the width of glitch.
FIGS. 31A and 31B show a relation between the light spot and the track, together with a waveform of tracking error signal TES in the phase difference (DPD) method in the structure shown in FIGS. 27 and the embodiments of the invention. Tracking error signal TES, which is obtained by using the signal processing circuit and the optical pickup having the structure disclosed in the foregoing publication, has a substantially linear form as shown in FIG. 28H, in which the level is 0 when light spot 124 is located immediately on a center of a track, and has the polarity corresponding to the direction of lateral shift (off-track) of the light spot 124 from the same track. In connection with a plurality of tracks, as shown in FIGS. 31A and 31B, the linear signal waveform appears for each track, and the level of 0 is likewise attained when light spot 124 is located between the tracks. Therefore, the signal have a sawtooth waveform having cycles corresponding to the tracks as a whole, as shown in FIG. 31B.
FIGS. 32A and 32B show a relation between the tracking error signal and the direction, in which the light spot is to be driven for the tracking servo-control, in the structure shown in FIG. 27.
For performing the tracking servo-control with the tracking error signal, which exhibits the sawtooth waveform corresponding to the tracks and has the polarities shown in FIG. 31B, a tracking servo-control system may have a structure provided with an unillustrated drive portion (generally called an tracking actuator) which drives objective lens 104 in such a manner than the position of light spot 124 moves selectively in the directions indicated by arrows A and B in FIG. 32A in accordance with the positive and negative polarities of tracking error signal TES, respectively.
For operating the optical pickup, it is necessary to employ not only the tracking servo-control for accurately moving light spot 124 to follow the tracks but also the track traversing operation called a "track jump" or a "track search" for moving light spot 124 to traverse the track in order to search and reproduce arbitrary information on disk 105 and, in other words, for moving the position of light spot 124 to an intended track in the radial direction of disk 105.
FIGS. 33A-33E shows a principle of detecting the number of traversed tracks and the traversing direction based on the tracking error signal and the reproduced information signal in the prior art. In order to improve the accuracy of the track traversing operation, tracking error signal TES can be referred to. More specifically, as shown in FIGS. 33A and 33B, attention is given to the fact that tracking error signal TES on the track crosses (zero-crosses) 0 level. Based on this fact, processing is performed to count the number of rising edges or falling edges of a signal in FIG. 33C which is prepared by binarizing tracking error signal TES, e.g., by a comparator. The count thus obtained represents the number of tracks which light spot 124 traversed.
Since tracking error signal TES performs the zero-crossing also at a position between the neighboring tracks, it is desired to refer additionally, at the same time, to a reproduced information signal RF for more accurate counting. For this additional reference, such a fact is utilized that signal RF in FIG. 33D located on the track has a large amplitude, and processing is performed to produce a signal shown in FIG. 33E, which attains logical level H when the envelope amplitude of signal RF exceeds a predetermined value (binarizing level). The edges of the signal in FIG. 33C produced by binarizing tracking error signal TES are counted only when the signal in FIG. 33E is at level H. In this manner, the above additional reference can be performed.
The traversing direction of light spot 124 with respect to the track (row of information pits 125) can be determined using the foregoing signals. For example, this direction can be determined by detecting which of the levels H and L does the signal in FIG. 33C attain at the rising edge of the signal in FIG. 33E. If the signal in FIG. 33C is observed and determined to be at level L at the rising edge of the signal in FIG. 33E, it can be found that light spot 124 traverses the track rightward in FIG. 33A. The change (edge) in the signal in FIG. 33C may be determined when the signal in FIG. 33E is at level H, so that the traversing direction can be determined based on the detected edge, i.e., the rising or falling edge. In this method, if the rising edge of the signal in FIG. 33C is detected when the signal in FIG. 33E is at level H, it is determined that light spot 124 traverses rightward in FIG. 33A.
According to the phase difference method (DPD method), tracking error signal TES has a sawtooth waveform as shown in FIG. 31B owing to the relative position between light spot 124 and the tracks in the direction across the tracks on the information record surface. By using this, the tracking servo-control for controlling light spot 124 to follow accurately the tracks is allowed. Also, when light spot 124 traverses the tracks for searching an intended track, the number of the traversed tracks and the traversing direction can be detected for increasing the accuracy of the track traversing operation of light spot 124. These have been described above.
For producing tracking error signal TES by the phase difference method (DPD method), as described before, it is necessary to extract low-frequency components of pulses (which will be referred to as "phase difference pulses" hereinafter) corresponding to the phase difference, which appears between two signals and corresponds to an amount of relative shift between light spot 124 and the track center. Properties (time constant) of the LPF, integrator or the like used for the extraction are fixed in the prior art.
When the phase difference method (DPD method) is to be used for obtaining tracking error signal TES, therefore, it is difficult in the prior art to satisfy both the operation for tracking servo-control and the track traversing operation. In other words, individual losses are produced depending on the manners of setting the time constant of the LPF or integrator, so that it is difficult to satisfy both the foregoing operations at the same time. Individual losses caused by setting the time constant to a certain value will be discussed below. This will clarify disadvantages in the prior art. As an example, it is now assumed that the time constant is set to a relatively small value, and individual losses will be successively discussed below.
FIGS. 34A and 34B show frequency characteristics of a gain and a phase of a primary LPF in the prior art. FIGS. 36A-36C show erroneous detection of the number of tracks traversed by the light spot and the traversing direction, and particularly the erroneous detection caused when the track traversing speed of the light spot is increased.
The first advantage achieved by setting the time constant to a small value is increase in stability owing to increase in phase margin of the tracking servo-control system.
For example, if the primary LPF is used for averaging processing of the phase difference pulse, the characteristics shown in FIGS. 34A and 34B are exhibited. At a pole frequency f.sub.LPF (normally set to tens of kilohertz) of the primary LPF, the gain is -3dB based on that with DC as shown in FIG. 34A, and the phase is -45 degrees as shown in FIG. 34B. At frequencies lower than the pole frequency f.sub.LPF, the gain and phase approach 0 dB and 0 degree, respectively. At frequencies higher than the frequency f.sub.LPF, the gain and phase asymptotically approach -.infin. dB (gain is 0 in absolute value) and -90 degrees, respectively.
As is well known, such a design is employed for stabilizing the servo-control system that a phase margin Om (i.e., a margin until the phase reaches -180 degrees at a cut-off frequency fc, in other words, at a frequency at which the gain is 0 dB) can be from about 30 to about 50 degrees owing to phase lead compensation (or differential calculus compensation). If the foregoing phase lag of the LPF is large, this phase margin significantly decreases so that the tracking servo-control system is liable to become unstable. However, if the pole frequency f.sub.LPF is set to a high value (i.e., the time constant is set to a small value), the phase lag decreases, and reduction in phase margin is also suppressed, so that the stable operation of the tracking servo-control system can be easily performed.
The second advantage achieved by employing the small time constant of the LPF or the integrator is as follows. In the track traversing operation of light spot 124, and particularly in the track search for which light spot 124 traverses a large number of tracks at a high speed, an error in determination of the traversing direction is unlikely to occur.
In the track traversing operation, it is generally necessary to detect the track traversing direction for controlling acceleration and deceleration of the relative speed of light spot 124 with respect to the track. The principle thereof is shown in FIGS. 33A-33E. Description will now be given on an influence in the case that the time constant is large and the phase lag (time lag) occurs in tracking error signal TES with reference to FIGS. 36A--36E and FIGS. 33A-33E.
When the track traversing speed of light spot 124 is low and therefore the repetition frequency of the sawtooth waveform of tracking error signal TES is low, or the foregoing time constant is small, the phase lag in tracking error signal TES during the track traversing does not particularly cause a problem. The number of traversed tracks and the traversing direction can be detected as already described with reference to FIGS. 33A-33E.
When the track traversing speed of light spot 124 is high and the repetition frequency of the sawtooth waveform of tracking error signal TES is high, the phase lag and reduction in amplitude due to the integrator or LPF gradually become remarkable in tracking error signal TES if the time constant is large. In this case, signal RF, tracking error signal TES and binary signals obtained therefrom exhibit relation shown in FIGS. 36A--36E which are different from those in FIGS. 33A-33E. In FIG. 36B, tracking error signal TES having a waveform represented by a solid line has a smaller amplitude and a delayed phase compared with the original waveform represented by a broken line. Therefore, the signal in FIG. 36C produced by binarizing it has edges shifted from those of the original waveform. If tracking error signal TES in FIG. 36B has a small amplitude, binarizing thereof may not be accurately performed, resulting in such phenomena that the position of the edge is shifted to a large extent or it is impossible to obtain the signal itself in FIG. 36C by binarizing signal TES. Therefore, even if the binary signal in FIG. 36C produced by binarizing tracking error signal TES in FIG. 36B is sampled at rising or falling of the signal in FIG. 36E produced by binarizing the envelope of signal RF in FIG. 36D with a predetermined level, the level of the sampled signal in FIG. 36C is instable, resulting in erroneous determination of the traversing direction of light spot 124.
Determination of the traversing direction of the light spot is required, for example, immediately after the start of the track search operation or immediately before the end of the same, and more specifically in such a case that the speed at which light spot 124 is driven with respect to the tracks may not be large compared with the speed of traversing in the opposite direction caused by decentering or eccentricity of the disk, and these relative speeds of the light spot 124 with respect to the tracks may be inverted (and therefore the relative moving direction may be inverted). While the moving or driving speed of light spot 124 with respect to the track is sufficiently high, the above inversion of the relative speeds cannot occur, so that the determination of the direction is not necessary, and the track traversing can be appropriately performed without referring to tracking error signal TES. If a large eccentricity is present at the disk, a high disk rotation speed is employed for fast data transfer, or a small track pitch is employed for improving the record density, the aforementioned relative speed due to the eccentricity at the disk is large, and it is necessary to perform the direction determination until the speed of moving light spot 124 with respect to the tracks exceeds this relative speed. Therefore, if a large time constant is employed, the direction may be erroneously determined due to the phase lag and the reduction in amplitude of tracking error signal TES.
However, if a small time constant of the LPF or integrator is employed, influences causing the phase lag and amplitude reduction of tracking error signal TES can be small even in the above situation, so that the error in direction determination can be reduced. This is the second advantage which can be achieved by employing a small time constant.
Meanwhile, a first disadvantage caused by employing a small time constant of the LPF or integrator is that defects on the disk or the like tends to disturb tracking error signal TES so that the accuracy in tracking servo-control is liable to be impaired.
When defects are present at the disk, diffracted or reflected light beams at light spot 124 radiated onto the track are disturbed so that abnormal or irregular pulses are mixed into the phase difference pulses. The width of such abnormal or irregular phase difference pulses and the frequency of occurrence thereof depend on the degree, extent and others of the defects. If the time constant of the LPF or integrator is small (i.e., if diffraction frequency f.sub.LPF is high in LPF), the abnormal or irregular phase pulses are not sufficiently averaged, and a irregularity occurs on tracking error signal TES so that the accuracy of the tracking servo-control system deteriorates. Particularly, if a large defect or the like is present at the disk, light spot 124 is fully shifted from the track, resulting in a significant problem.
A second disadvantage caused by employing the small time constant of the LPF or integrator is as follows. It is difficult to detect a midpoint between the tracks which is desired to be detected in the track traversing operation of light spot 124, and particularly for the track jump operation performed by traversing several tracks.
In the tracking servo-operation, and, in other words, when light spot 124 is following the track, the defects or the like at the disk disturb tracking error signal TES as already described. However, as light spot 124 is shifted from the center line of the track to a higher extent, a disturbance tends to occur at the waveform of tracking error signal TES produced by the phase difference (DPD) method to a higher extent even if no defect is present on the disk.
FIGS. 37A-37D show a relation between the off-track of the light spot and the disturbance at the waveform of the tracking error signal in the circuit structure shown in FIG. 27. FIGS. 37A-37C show a transition from the on-track state to the off-track state of the light spot, and FIG. 37D shows tracking error signal TES issued from the circuit in FIG. 27 in accordance with the transition or change of the states in FIGS. 37A-37C. According to the phase difference (DPD) method, as already described, tracking error signal TES is produced based on the phase difference pulses corresponding to the timings of changes of the light quantities at the right and left regions of light spot 124 divided in the tracking direction while light spot 124 is passing over information pits 125. As shown in FIGS. 37A-37C, when light spot 124 is significantly shifted from the center line of the track (row of information pits 125) and approaches the midpoint between the tracks, a region of light spot 124 covering information pit 125 decreases in accordance with the above approach, and it becomes difficult to detect from the reflected light beams that light spot 124 passed over information pit 125. Therefore, abnormal or irregular pulses appear in the phase difference pulses corresponding to the amount of shift between light spot 124 and the track, i.e., the off-track amount, so that the waveform of tracking error signal TES in FIG. 37D is gradually disturbed. In particular, when light spot 124 is located at the midpoint between the neighboring tracks and covers slightly both the left and right adjacent tracks, abnormal and irregular pulses are most likely to occur at the phase difference pulse due to a minute change in the light beams reflected or diffracted by information pits 125 at the left and right tracks neighboring thereto, so that tracking error signal TES in FIG. 37D is disturbed to the highest extent.
Detection of the position between the tracks is usually unnecessary when light spot 124 traverses tens or hundreds of tracks or more a time. However, in the jump operation of jumping several tracks or less, it is necessary to detect the position between the tracks for determining the timing of acceleration or deceleration of light spot 124 and thereby increasing a success rate. In an extreme case, both the acceleration and deceleration of light spot 124 are required only during movement over one track for performing one-track jump operation, which is often employed in still jump. For this purpose, it is necessary, for stabilizing the jump operation, to detect the midpoint between the tracks as the switching point between the acceleration and deceleration of light spot 124 even if a certain error is present.
However, if tracking error signal TES is disturbed between the tracks as shown in FIG. 37D, the midpoint between the tracks cannot be detected accurately, so that light spot 124 cannot be accelerated and decelerated stably at accurate timings and therefore a failure is liable to occur in the track jump operation (traversing).
If the time constant of the LPF or integrator is large, the aforementioned abnormal and irregular pulses in the phase difference pulses are strongly averaged, so that a disturbance at tracking error signal TES at and around the midpoint between the tracks is reduced, and the midpoint between the tracks can be easily detected from the zero-cross point. However, if the time constant is small, tracking error signal TES at and around the midpoint between the tracks is disturbed to a large extent as shown in FIG. 37D, and it is difficult to detect the midpoint between the tracks from the zero-cross point.
As described above, if the time constant of the LPF or integrator is small, the foregoing two advantages and two disadvantages arise in the circuit shown in FIG. 27. On the other hand, if the time constant of the LPF or integrator is large, advantages and disadvantages opposite to the above naturally arise. Determination of the value of the time constant in each tracking control device is a matter of design. In the conventional tracking control device for producing tracking error signal TES by the phase difference (DPD) method, the characteristics of the low-pass portion, i.e., characteristics of the LPF or integrator are set commonly to quite different two operations, i.e., the tracking servo-control and the tracking traversing operation. Therefore, it is impossible to overcome the disadvantages while maintaining the foregoing advantages. Thus, the tracking control device in the prior art can satisfy, at the same time, only two of the four items, i.e., (1) stability of the tracking servo-control system, (2) accuracy of the direction determination during track search, (3) accuracy of the tracking servo-control with respect to defects on the disk or the like, and (4) detection of the midpoint between the tracks during track jump. Therefore, the prior art suffers from such a problem that a trade-off relationship is unavoidably present between the accuracy of the track traversing and stability/accuracy of the tracking servo-control.